Particular embodiments generally relate to power amplifiers.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts an example of a power amplifier (PA) 100. Power amplifier (PA) 100 is coupled to a wideband transformer 101 that includes a transformer 102 and a load 104. Power amplifier 100 may be part of a wireless transmitter. In one example, a signal from power amplifier 100 is coupled through transformer 102 and transmitted through an antenna.
Power amplifier 100 includes a first amplifier 108a, an inductor-capacitor (LC) resonant tank 110, a second amplifier 108b, and a resistor R1 in a feedback loop. Power amplifier 100 may become unstable. An input circuit 106 provides an input signal to power amplifier 100.
A dominant pole is introduced by amplifier 108a and LC resonant tank 110. This introduces a phase shift of around 90° (at a unity gain point) at for the signal output by power amplifier 100. Also, a phase shift introduced by amplifier 108b and transformer 102 may be less, such as 30°. To have a stable power amplifier, a total phase shift for power amplifier 100 should be less than 180°. Thus, a phase shift introduced by the feedback loop should be less than 60°.
The input impedance is shown as impedance Z_input and in this case, is a parasitic capacitance of a transistor in amplifier 108a. The parasitic capacitance is modeled as a parasitic capacitor Cp. Because the feedback impedance Z_feedback is a resistance, the resistor-capacitor combination may introduce a phase shift that is greater than 60°. This may cause power amplifier 100 to be unstable and the signal may oscillate. Also, the resistor-capacitor combination may create a pole that is within the working bandwidth of the wireless transmitter. This may alter the gain characteristics for power amplifier 100.